Case/PSU... #1 is that you have a PSU with a 24 pin motherboard plug. That Rosewill case/PSU, I'm guessing it's the $22 one listed? That is NOT 24 pin, so do not get it.
I'd highly recommend the Biostar Tforce6100 board.
Zap's mini review
It's only $9 more than the ASRock board and it can overclock just about as well as the DFI boards. Also, it has "Smart Fan" settings that you can adjust in BIOS to reduce the CPU fan noise. I think that $9 will gain you a few hundred MHz worth of overclocking. BTW, the Biostar Geforce6100-M7 is NOT the same thing. It does have better overclocking than pretty much all other mATX Geforce6100 chipset based boards but still is a noticeable bit behind the Tforce6100 (I own both) and doesn't have "Smart Fan."
RAM... any will work and you have your choice of 2x512MB or single 1GB modules for under $70 shipped. BTW, avoid the cheap "AMD only" 1GB modules.
How about this
$15 + $13 shipping SkyHawk case with 120mm exhaust fan and no PSU?
This Fortron 300W unit is
$23 + $7 shipping and is 24 pin and dual rails (16A + 16A).
Sempron 2600+ is $74 shipped.
Biostar Tforce6100 is $74 shipped.
CPU $74
Mobo $74
RAM $70
PSU $30
Case $28
Total $276 shipped
Okay, that is $26 above your target price but is tenfold better because of:
- 120mm case exhaust fan (but may not come with one

)
- Fortron PSU, guaranteed many times better than in that $22 Rosewill case plus has 24 pin that you NEED
- Motherboard will clock that 1.6GHz Sempron 2600+ to 2.4-2.5GHz, something that the ASRock will not get close to, if it can overclock at all.
My own Sempron 2600+ hit 2.52GHz Prime stable with some major voltage - needed because I had the older D0 core. If the new one is an E3 or E6 core, may be able to go higher on less voltage.
If you absolutely, absolutely need to shave cost, get the Biostar Geforce6100-M7 board. It was only able to hit 2.2GHz or so (forgot exact speed) with my same Sempron but that still puts it above the other boards except for the Tforce6100 and will cost the same as the ASRock. Your total would be $267 shipped price, only $17 above projected budget.